Electron beam inspection apparatus

ABSTRACT

An electron beam inspection apparatus in which the order of inspection is determined to shorten the inspection time is disclosed. The order of inspection is determined by minimizing the total of the moving time and the inspection time as well as by simply optimizing the covered distance. At the time of preparing a recipe to determine the inspection points and the order of inspection, the sequence of a series of inspection points sequentially inspected is changed to optimize the order of inspection. Not only the sequence which minimizes the covered distance is determined but also the order of inspection of the inspection points is optimized in accordance with the charged state, warping of the wafer, the delivery position and other situations.

This application is a Continuation of U.S. application No. 11/099,688,filed Apr. 6, 2005, now U.S. Pat. No. 7,256,400, claiming priority ofJapanese Application No. 2004-114859, filed Apr. 9, 2004, the entirecontents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to an electron beam inspection apparatus in whicha plurality of inspection points are moved to shorten the inspectiontime.

The inspection points are arranged by the user arbitrarily and hence inan inefficient order of inspection. To assure efficiency, a method isalso available to arrange the inspection points in the form of bellows.In this arrangement in bellows form, however, the inspection time is notnecessarily minimized. The inspection time is increased conspicuouslydue to the increased distance in the inspection of a wafer having adiameter as large as 300 mm. Therefore, the optimization of the order ofinspection is required and has not been realized due to the failure toestablish an algorithm which completes the calculation in practicalcalculation time.

SUMMARY OF THE INVENTION

The object of this invention is to provide an electron beam inspectionapparatus in which the calculation function for determining the order ofinspection is realized to shorten the inspection time. The calculationshould desirably be completed in a practicable range of not longer than30 seconds. The order of inspection is determined by shortening thetotal time including the moving time and the inspection time as well asthe distance covered.

According to this invention, there is provided an electron beaminspection apparatus wherein the function to optimize the order ofinspection is realized at the time of preparing a recipe to determinethe inspection points and the order of inspection. The inspectionapparatus has the function not only to determine the order of inspectionfor shortening the distance covered but also to optimize the order ofinspection of inspection points in accordance with the carrying-outposition and other prevailing situations. After determining theinspection points, the order of inspection is changed using the orderchanging function. The formula used to change the order of inspection isautomatically selected in accordance with the number of inspectionpoints involved.

As described above, the inspection time is shortened by changing theorder of inspection of the inspection points in an electron beaminspection apparatus thereby to contribute to an improved throughput.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the process for preparing a recipe.

FIG. 2 shows an outline of the process for calculating an optimum route.

FIG. 3 shows the process of calculation to determine an optimum route.

FIG. 4 shows the process of calculation to determine an optimum route.

FIG. 5 shows the normal order of inspection.

FIG. 6 shows the order of inspection in the shortest distance.

FIG. 7 shows the order of inspection in bellows type (return type).

FIG. 8 shows an outline of a measuring SEM.

FIG. 9 shows a route determined taking the alignment end point and thewafer carrying-out position into consideration.

FIG. 10 shows an optimum route for each of two wafer carrying-outpositions.

FIGS. 11A and 11B show an example in which an optimum route is acquiredfor each of two wafer carrying-out positions taking the alignment pointinto consideration.

FIGS. 12A, 12B and 12C show an example using an optical microscope foralignment.

FIGS. 13A and 13B show the calculation of the moving time to determinethe optimum route.

FIGS. 14A and 14B show an example in which the time other than themoving time can be shortened by the route.

FIG. 15 is a diagram showing a general configuration of a scanningelectron microscope according to an example of the invention.

FIG. 16 shows the selection by calculation of an optimum route.

FIG. 17 shows a screen to store the route information.

FIG. 18 shows an example of the trapezoidal control operation of thestage speed.

DESCRIPTION OF THE INVENTION

A basic operation flow of the invention is shown in FIG. 1. In theconventional recipe preparation process, the inspection points aredetermined arbitrarily by the user, and the apparatus performs theinspection in accordance with the procedure thus determined. Accordingto this invention, the recipe begins to be prepared (S0001), thecontents of inspection are determined (S0002) and the inspection pointsare determined (S0003), after which an optimum route calculation menu(102) is selected (S0004) from an optimum route calculation and selectscreen (100) shown in FIG. 16. Upon selection (S0004) of the optimumroute calculation menu (102), the calculation of the optimum route isstarted (S0005). The optimum route is calculated automatically. Aftercalculation of the optimum route, the order of inspection of the optimumroute is indicated on an inspection point list (101). The change in theorder of inspection is selected (S0006), and an OK button (106) isselected (S0007) on the route information storage confirmation screen(105) thereby to determine the optimum route. No route information isstored in the case where the “cancel” button (107) is selected. StepsS0004 to S0007 represent new steps added by the invention. The selectionof the optimum route calculation start menu, the route informationstorage confirmation, etc. are conducted by use of an input device (52)while watching a CRT (53) shown in FIG. 15. The route information,together with the recipe information, are stored in a storage unit (51).

The calculation process of the optimum route is shown in FIG. 2.Strictly, the optimum route cannot be determined without calculating allthe combinations of the inspection orders. With the increase in theinspection points, however, the number of combinations increases and allthe combinations cannot be calculated within a practical calculationtime. According to this invention, the calculation algorithm is changedin accordance with a predetermined number of measurement points. In thecase where the number of measurement points is smaller than n1 and n2,all the route combinations are calculated to calculate the optimum routestrictly (S0008, S0009, S0010). In the case where the measurement pointsexceed n1 and n2, on the other hand, an optimum route is calculatedapproximately by the optimization calculation algorithm or thebellows-type route calculation (S0011, S0012).

An example of the algorithm to calculate an optimum route is shown inFIG. 3. According to this algorithm, continuous inspection points in apredetermined order of inspection are changed and the inspection time iscompared. Then, the order of inspection which shortens the inspectiontime is selected. First, an initial value n is set to 1 (S0013). Then,the calculation result of the total covered distance is stored as A1(S0014). The inspection order n is replaced with the inspection ordern+1 and the total covered distance is calculated, the result of which isstored as A2 (S0015). In the case where A1 is larger than A2, theinspection order n is replaced with n+1 so that n is set to 1 and theprocess is returned to S0014 (S0016, S0017). In this case, n is set to 1at S0017 and the process is returned to S0014. Nevertheless, n may bealternatively set to n−1 at S0017. In the case where A2 is larger thanA1, on the other hand, n is set to n+1, and in the case where ncoincides with the number of inspection points, the inspection iscompleted. Otherwise, the process is returned to S0014 (S0018, S0019).

The route after calculation follows exactly the same order as beforecalculation or the order with a shorter inspection time than beforecalculation. The calculation time is also comparatively shortened. Sinceonly the continuous inspection points are replaced for comparison,however, the route after calculation considerably depends on the routebefore calculation. Depending on the manner in which the route beforecalculation is selected, therefore, the optimum route may fail to bedetermined. For this reason, this algorithm is more effectively usedonly after determining an approximately optimum route by anotheralgorithm to determine the optimum route.

Next, another example of the algorithm to calculate the optimum route isshown in FIG. 4. In this calculation method, the starting and endingpoints are defined first of all, and using them as an inspection route,inspection points are added (inserted) in the inspection route. Thecandidate points to be added (inserted) are those not yet added to theroute, and inspection points minimizing the inspection time for thewhole route are inserted in the order minimizing the inspection time. InFIG. 4, points u1−1, u1, u1+1, u1+2 are already added to the inspectionroute, and points v, v1, v2, v3 candidates to be added to the route.Specifically, the following procedure is followed.

Assume that the input is a weighted complete graph G at the position pwith the side weight satisfying the triangle inequality w(u, v)+w(u,w)≧w(v, w) at arbitrary three input points u, v, w. The output isassumed to be a Hamilton closed path C of appropriate weight.

-   1. Select uEV (G), and regard u as 1 minus closed path C1. (i←1)-   2. If i=p, end as C=Cp.-   3. If i‥p, select a point v where w(u1, v)+w(v, u1+1)−w(u1, u1+1) is    minimum out of points adjacent to continuous points u1, u1+1 on Ci    but not on C.-   4. By setting i←i+1, repeat the processes 2 to 4 above.

As a result, in the case of FIG. 4, w(u1, v)+w(v, u1+1)<w(u1, v1)+w(v1,u1+1), and therefore v is inserted in the route.

This calculation is repeated until all the inspection points are addedto the route. This method, as compared with the method of FIG. 3, hasthe feature that the calculation time increases with the number ofinspection points. Since this method is hardly affected by the initialroute, however, a route comparatively near to the optimum route can becalculated. An effective method is to use this algorithm and then thealgorithm of FIG. 3.

This invention is not limited to the aforementioned algorithms, but canuse, for example, TSP (traveling salesman problem) or the nearestinspection algorithm in public domain as a method of calculating theoptimum route.

The result of calculation of the optimum route is explained withreference to the examples shown in FIGS. 5 to 7. FIG. 5 shows a case inwhich the inspection is conducted in the ascending order of both X and Ycoordinates. This order of inspection is often determined arbitrarily bythe user. In this inspection order, the total distance betweeninspection points is not shortest, and therefore the total inspectiontime can be reduced by changing the order of inspection. FIG. 6 showsthe result of determining the optimum route using the functions of theinvention. The optimum route is not necessarily determined uniquely, andthis route is an example of the optimum route. The average covereddistance is 3.66 chips for the route shown in FIG. 5, while the figurefor the optimum route is 2.31 chips or about 40% shorter. This exampleassumes that the covered distance is proportional to the inspectiontime. In actual calculations, however, parameters other than the covereddistance can be used, as described in detail later. FIG. 7 shows anexample of a bellows-type route. The inspection is conducted in theascending order of X coordinate as in FIG. 5, while the ascending orderand the descending order are alternated with each other in Y direction.As a result, the number of reciprocations in Y direction can be reduced.Thus, the total covered distance and hence the total measurement timecan be reduced. The average covered distance along the route shown inFIG. 7 is 2.87 chips. In this case, as compared with FIGS. 5 and 7, theaverage covered distance is reduced by about 20%.

This invention is effectively applicable to the measuring SEM and thereview SEM in which as shown in FIG. 8, the inspection is conducted on asample 8 moved by moving an X table 2 and a Y table 3 controllable whileat the same time radiating an electron beam 9. Also, apart from theinspection apparatus using an electron beam, the invention iseffectively applicable to a case in which the inspection range is narrowand the inspection points are moved while moving the sample 8. Althoughthe XY stages are used to move the sample 8 in the case underconsideration, the invention is also applicable to a Rθ stage having arotary shaft and an axis to move the stage or a case in which the stagemoves along one axis and the electron beam along an axis perpendicularthereto. Further, in FIG. 8, 1 denotes a base, 4 denotes an X-axismotor, 5 denotes a Y-axis motor, 6 denotes a sample chamber, and 7denotes a main body.

FIGS. 9 to 11B show an application of the invention to the measuringSEM. In the measuring SEM, before moving the inspection points toconduct the inspection, an image is recognized at a predeterminedalignment point to adjust the wafer coordinate. The moving time from thealignment point before moving to the inspection points, therefore, isalso a factor contributing an increased total measurement time. Also,after the last inspection session, the sample 8 is moved to a wafercarrying-out position before moving to a preliminary exhaust chamber. Inoptimization of the total moving time, therefore, the moving time fromthe last inspection point to the wafer carrying-out position is alsopreferably included in the calculation. FIG. 9 shows a case in which themovement from the alignment end point to the inspection point and themovement from the last inspection point to the wafer carrying-outposition are also included in the calculation. In the optimum route ofFIG. 9( a) in which only the inspection points are considered in thecalculation, the average covered distance is 2.77 chips, while theaverage covered distance is 2.17 chips or about 20% smaller for theroute shown-in FIG. 9( b) in which the movement from the alignment endpoint to the inspection points and the movement from the last inspectionpoint to the wafer carrying-out position are included in thecalculation.

FIG. 10 shows an application to a plurality of preliminary exhaustchambers. In this apparatus, the optimum route for the carrying-outposition 1 is not necessarily the optimum route for the carrying-outposition 2. In the route shown in FIG. 10( a), the average covereddistance is 2.17 chips for carrying out to the carrying-out position 1,and 2.68 chips for carrying out to the carrying-out position 2. In thecase where the route shown in FIG. 10( b) is followed, however, thecovered distance is 2.17 chips for the carrying-out position 2. In thisway, a plurality of optimum routes are prepared using the algorithm ofthe invention, and in accordance with the wafer carrying-out positionfor inspection, an appropriate one of the optimum routes is selected.Thus, the optimum route can be inspected in keeping with the conditions.

The example shown in FIGS. 11A and 11B represents a case in which thesequence of a plurality of alignment points is also taken into accountfor optimization. A plurality of alignment points are generally used. Inthe measurement sequence from the wafer carrying-in position (normallythe same as the carrying-out position) through all the alignment pointsand all the inspection points to the carrying-out position, therefore,the route shortest in inspection time is calculated as an optimum route.

In the case where alignment is carried out using an optical microscope,the calculation is further required taking the offset intoconsideration. As shown in FIG. 12C, the optical axes of the opticalmicroscope and the electron microscope are offset from each other, andin the case where a point AL1 is inspected under the optical microscope,the point AL1 is moved to the axial position of the optical microscope.Under this condition, a chip OL1 is located at the position on theoptical axis of the electron microscope. This chip is moved to OL1 interms of the coordinate system observed under the electron microscope.In the coordinate of the alignment points for calculating theoptimization, therefore, the offset is required to be automaticallycalculated while the moving position is regarded as OL1 for calculation.The offset amount is unique to each apparatus, and defined in advance.Therefore, this distance can be used for conversion. In the case wherethe alignment is conducted using the same chip AL1 under electronmicroscope, on the other hand, as shown in FIG. 12A, conversion from thechip AL1 is required, while the alignment using the optical microscoperequires the conversion to the position of OL1 as shown in FIG. 12Bbefore optimization calculation. In this way, the optimum route iscalculated in FIG. 12B.

The foregoing explanation concerns a case in which the total inspectiontime is minimized by minimizing the total covered distance. In thiscase, the moving time T is proportional to √((ΔX)²+(ΔY)²) in FIG. 13A.In the stage adapted to move in X and Y directions independently of eachother, the total moving time is that along X or Y direction, whicheveris longer. On such an XY stage, therefore, the moving time in Xdirection and the moving time in Y direction are calculated from thecoordinates before and after movement, so that the moving time isdetermined from the longer one of the distances. In such a case, themoving time T is proportional to MAX(ΔX, ΔY). Generally, in FIG. 13B,assume a weighted complete graph G at the input position p with the sideweight satisfying the triangle inequality w(u, v)+w(u, w)≧w(v, w) atarbitrary three input points u, v, w. Then, the optimum route can becalculated using the optimization algorithm. The moving time, etc. whichsatisfy the equation above, can be used for calculation.

FIG. 14 shows a case in which the parameters other than the covereddistance are controlling over the measurement time. In the case wherethe sample surface is known to be charged like a contour and the timerequired for correction is longer than the moving time of the sample,for example, the total inspection time can be minimized by conductingthe inspection in such an order as to minimize the correction amount.The optimum route in terms of the covered distance shown in FIG. 14A isaccompanied by a total of eight potential variations, while the optimumroute along the equipotential line shown in FIG. 14B has a total of twopotential variations. Also in this case, the optimum route can becalculated following the procedure according to the invention byexpressing the correction time due to the potential difference betweenthe inspection points in numerical values.

The process of minimizing the total inspection time with the waferheight change Δh as a parameter is explained with reference to FIGS. 15and 18. Generally, the height of each inspection point on a wafer 20 isvaried by a least 100 μm even on the stage 25, and since the focal depthof the SEM is less than 1 μm, the focusing is impossible. In view ofthis, a height detecting laser 26 is applied to the wafer 20, and thereflection thereof is detected by a laser detector 27 to measure theheight. Based on the height information obtained by the measurement, thecurrent for an objective lens 17 is controlled by an objective lenscontrol power supply 33 through a computer 50 to attain the focusing.The objective lens 17 reacts to the set current with a predeterminedtime constant (delay). Specifically, the larger the change of thecurrent, the longer the time required to set a target focal point.Further, while the objective lens current is directly changed, the imageis picked up and the sharpness is determined. Thus, the automaticfocusing operation (AF) is conducted by setting an objective lenscurrent associated with the highest sharpness.

Further, in FIG. 15, 11 denotes a cathode, 12 denotes a first anode, 13denotes a second anode, 14 denotes an electron beam, 15 denotes a firstconvergence lens, 16 denotes a second convergence lens, 18 denotes anaperture plate, 19 denotes a scanning coil, 21 denotes an orthogonalelectromagnetic field (E×B) for separating secondary signals, 22 denotesthe secondary signals, 23 denotes a detector for secondary signals, 24denotes an amplifier, 30 denotes a power supply for controlling a highvoltage, 31 denotes a power supply for controlling the first convergencelens, 32 denotes a power supply for controlling the second convergencelens, 34 denotes power supply for controlling the scanning coil, 35denotes an image memory, 41 denotes a power supply for controlling analigner for the objective lens, and 61 denotes an aligner for theobjective lens.

In executing the recipe, let Ts be the stage moving time to move thestage to an inspection point, and T1 the reaction waiting time due tothe objective lens current width from the previous inspection pointbased on the laser measurement of the wafer height at the particularposition. The processing time Tt from a measurement session to the nextmeasurement session is given asTt=Ts+T1+Tapwhere Tap is the sum of the pattern recognition time and the AFexecution time and substantially constant. In the case where the stagespeed is controlled in a simple trapezoidal fashion, Ts which is afunction of the distance d between the two measurement points is givenas, when d<Vmax*Vmax/2*(1/α1+1/α2),Ts=√(2d*(1/α1+1/α2)) and, when d≧Vmax*Vmax/2*1/α1+1/α2)Ts=d/Vmax+Vmax/2*(1/α1+1/α2)where Vmax is the maximum stage speed, α1 the acceleration, and α2 thedeceleration. Also, using the height change Δh, T1 is expressed asT1=A*exp(Δh/τ)where A and τ are constants unique to the lens.

These time values are determined for each route so that the totalmeasurement time can be minimized at the time of optimizationcalculation.

The foregoing description concerns CD-SEM as an example. This invention,however, is not limited to the CD-SEM but applicable also to variouselectron beam inspection apparatuses with equal effect.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A scanning electron microscope for determining order of irradiationin irradiating an electron beam on one of a plurality of irradiationpositions on a sample after focusing is conducted at said one of theplurality of irradiation positions, wherein one of a plurality ofirradiation orders which a total of focusing time it takes for focusingthe electron beam at the irradiation positions in each of saidirradiation orders is shorter is selected.
 2. The scanning electronmicroscope according to claim 1, wherein the total of focusing time isdetermined on the basis of a total of potential variations between theplurality of irradiation positions.
 3. The scanning electron microscopeaccording to claim 1, wherein the total of focusing time is determinedon the basis of height changes between the plurality of irradiationpositions.
 4. A scanning electron microscope for determining order ofirradiation in irradiating an electron beam on a plurality ofirradiation positions on a sample adapted to move in X and Y directions,wherein a longer one of respective moving distances of the X and Ydirections between the irradiation positions is selected, and theselected ones of moving distances are compared respectively between aplurality of moving routes to select a shorter moving route.
 5. Ascanning electron microscope comprising a controllable sample stage anda control apparatus for controlling said sample stage so that each of aplurality of positions on said sample automatically moves to anirradiation point of an electron beam in a predetermined order, wherein:said sample stage is a XY stage to move said sample in X direction and Ydirection, and said control apparatus changes said predetermined orderbased on a condition where a summation of moving times between two ofsaid plurality of positions on said sample is shortened, wherein alonger time of moving times in X direction and Y direction is determinedas the moving time between two of said plurality of positions on saidsample.
 6. The scanning electron microscope according to claim 5,wherein said sample stage moves in X direction independently from in Ydirection.
 7. A scanning electron microscope comprising a controllablesample stage and a control apparatus for controlling said sample stageso that each of a plurality of positions on said sample automaticallymoves to an irradiation point of an electron beam in a predeterminedorder, wherein: said control apparatus changes said predetermined orderso that a calculated total value of processing times is shortened,wherein said processing time is calculated based on a waiting time of anobjective lens to react on a current for the objective lens, which isobtained from a height information of a position on said sample, andbased on a summation of moving times between two of said plurality ofpositions on said sample.
 8. The scanning electron microscope accordingto claim 7, wherein the processing time is a total value of eachsummation of said moving time between two of said plurality of positionson said sample, said waiting time of said objective lens to react onsaid current for said objective lens, a pattern recognition time and anauto focus (AF) execution time.
 9. A scanning electron microscopecomprising a controllable sample stage and a control apparatus forcontrolling said sample stage so that each of a plurality of positionson said sample automatically moves to the irradiation point of anelectron beam in a predetermined order, wherein: said control apparatuschanges said predetermined order so that a moving route becomes along anequipotential line indicating a distribution of electric potential ofsaid sample.
 10. A scanning electron microscope comprising acontrollable sample stage and a control apparatus for controlling saidsample stage so that each of a plurality of positions on said sampleautomatically moves to the irradiation point of an electron beam in apredetermined order, wherein: said control apparatus changes saidpredetermined order so that a summation of electric potentialdifferences between two of said plurality of positions on said sample isshortened.